Metabolic Cell Support (MC-S), now manufactured by Clinical Health Pty Ltd, is an over the counter immune supplement developed in Australia. Rather than using individual bioactive chemicals, a combined highly standardised multipolysaccharide enriched mix of medicinal mushrooms, the herb Astragalus mebranasis and ascorbic acid was formulated so as not to exclude important components of the original herb that may play a role in clinical outcomes. To examine the potential effect of MC-S on cancer, an in vitro proliferation study using diverse types of cancer cell lines was undertaken. Breast (MCF-7), prostate (DU145), melanoma (MM200) and colon (HT29) cancer cells were cultured under controlled standard conditions in the presence of MC-S at varying doses. After 48 and 72 hours of incubation, proliferation was measured using CellTiter 96 AQueous Non-Radioactive Cell proliferation assay kit. Results MC-S significantly inhibited the growth of breast, prostate, melanoma and bowl cancer cells, over a broad range of doses. Conclusions MC-S in this study was shown to inhibit cancer cell growth. This result, along with previous studies identifying immune stimulation properties, suggests that MC-S may offer additional benefit to standard cancer therapies.